To add a custom embed code or raw HTML, you will first need to set up a field specifically for this. Then you will need to add code to your website application to handle this new field.
1. Create a new field for the HTML
You have two options, using a Key Text field or a Rich Text field.
Using a customized Rich Text field (recommended)
This option is to set up a Rich Text field specifically for your custom code. You will need to configure the field to "Allow multiple paragraphs" and set the field to only allow preformatted text.
Or using the Key Text field
The option is to use a Key Text field. You can paste the code you need into it. Then it will output as plain text, which you can inject straight into your code.
This option will appear a one big string in the Prismic interface, so it won't look as good as the previous option.
When using this option, your code might also be encoded when you save the document. If using this method, you will likely need to unencode the content before adding it to your pages.
2. Add the new field to your website application
The next step is to update your website code to handle this new field. You will need to output this field as plain text straight into your template.
If you aren't a developer then you will likely need help from your dev team for this step.
If you have any questions about this, don't hesitate to reach out via the chat support!